Identifying the Mechanism of the Selective Hydrogenation of 1,3-Butadiene over the Well-Defined PdCu Alloy

PdCu bimetallic catalysts can enhance both selectivity and activity simultaneously in the selective hydrogenation of 1,3-butadiene, although the mechanism behind the enhanced performance of the PdCu catalysts remains insufficiently understood. The presence of mixed and disordered alloy phases on the bimetallic catalyst surface hinders the identification of the intrinsic reaction mechanism. In the current work, catalysts with well-defined Pd, Pd1Cu1, Pd1Cu3 alloy phases were controllably synthesized to investigate the selective hydrogenation performance of 1,3-butadiene. It is found that Pd1Cu1/SiO2 and Pd1Cu3/SiO2 exhibit not only higher intrinsic activity but also higher selectivity toward total butenes and 1-butene than Pd/SiO2 due to the electronic effect, which becomes more significant with increasing Cu/Pd. Furthermore, ideal Pd(111), Pd1Cu1(111), and Pd1Cu3(111) surfaces were modeled by DFT calculations to conduct theoretical studies. It is found that the electronic effect is more significant and the adsorption energy to butenes decreases with increasing Cu/Pd ratio. By analyzing the transition states of the elementary reactions in the reaction pathways, the reaction mechanisms are revealed on different surface sites. It is indicated that the bimetallic PdCu surfaces exhibit higher intrinsic hydrogenation activity and higher selectivity to total butene and 1-butene, in agreement with the experimental results.